Selective noncatalytic reduction

When NO destmction efficiencies approaching 90% are required, some form of post-combustion technology appHed downstream of the combustion 2one is needed to reduce the NO formed during the combustion process. Three post-combustion NO control technologies are utilized selective catalytic reduction (SCR) nonselective catalytic reduction (NSCR) and selective noncatalytic reduction (SNCR). 

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

Combustion modifications and postcombustion processes are the two major compliance options for NO., emissions available to utilities using coal-fircd boilers. Combustion modifications include low-NO burners (LNBs), overfire air (OFA), reburning, flue gas recirculation (FGR), and operational modifications. Postcombustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). The CCT program has demonstrated innovative technologies in both of these major categories. Combustion modifications offer a less-expensive appiroach. 

Postcombustion processes are designed to capture NO, after it has been produced. In a selective catalytic reduction (SCR) system, ammonia is mixed with flue gas in the presence of a catalyst to transform the NO, into molecular nitrogen and water. In a selective noncatalytic reduction (SNCR) system, a reducing agent, such as ammonia or urea, is injected into the furnace above the combustion zone where it reacts with the NO, to form nitrogen gas and water vapor. Existing postcombustion processes are costly and each has drawbacks. SCR relies on expensive catalysts and experiences problems with ammonia adsorption on the fly ash. SNCR systems have not been proven for boilers larger than 300 MW. 

Selective catalytic reduction (SCR) and selective noncatalytic reduction processes (SNCR) are widely employed in large industrial and utility boiler plants, as well as in municipal waste incineration plants and other combustion processes. They are used to complement mechanical improvements (such as low NOx burners and furnace design modifications) as an aid to reducing the emission levels of NOx, S02, and other noxious gases into the atmosphere. 

The selective Noncatalytic reduction (SNCR) process is a postcombustion NO reduction technology. NO is reduced through the controlled injection of a reagent, either ammonia or urea, into the combustion products of boiler, heater, or FCC regenerator. This process is typically applied on partial burn applications with a CO boiler (COB). 

The postcombustion systems used at power generating plants and factories are somewhat different. These systems remove nitrogen oxides from the waste gases jlue gases) processes using classified as selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). Oxides of nitrogen are also removed by some systems... 

Selective noncatalytic reduction systems make use of the tendency of certain compounds, ammonia (NH3) and urea (CO[NH2]2) in particular, to react with and reduce oxides of nitrogen ... 

Gaseous pollutants in combustion exhaust gases dry sorbent injection, semidry sorbent injection, wet sorbent injection, selective noncatalytic reduction of N()x (SNCR), selective catalytic reduction of NOx (SCR). 

Parallel reactions play an important role in chemical reaction systems that involve selectivity. An example is the selective noncatalytic reduction of NO (SNCR), which is a widespread secondary measure for NO control. In this process NO is reduced to N2 by injection of a reducing agent such as NH3 into the flue gas in a narrow temperature range around 1000°C. The process is characterized by a selectivity in the reaction pathways as shown by the parallel (global) steps... 

Fig. 13.6 Laboratory flow reactor experiments on selective noncatalytic reduction of nitric oxide by ammonia, with and without hydrogen addition [251]. Inlet composition NO = 225 ppm, NH3 = 450 ppm, O2 = 1.23% balance inert. Residence time 0.075 s atmospheric pressure.

Model-based boiler optimization schemes have proved successful in many power plant and industrial boiler applications. Successful NOx reduction through this kind of optimization can avoid or postpone large capital expenditures for low NOx burners, over-fire air modifications, and selective catalytic reduction/selective noncatalytic reduction (SCR/SNCR). 

Most of the existing processes for nitrogen oxide removal are chemically based requiring high temperature or expensive catalysts. The main techniques involve either selective noncatalytic reduction (SNCR) or selective catalytic reduction (SCR). SNCR uses ammonia for conversion of NO to N2 and H20 at elevated temperatures (550-850 K). SCR can use catalysts such as Ti02 with active coatings of V2Os and WO, . 

SNCR [Selective NonCatalytic Reduction] A generic term for processes that remove oxides of nitrogen from flue gases by noncatalytic chemical reactions. These include the reaction with ammonia at high temperature (1,300 to 1,900°C), and the reaction with urea. See NOxOut, SCR. 

R. Hemberger, S. Muris, K-U. Pleban and J. Wolfram, An Experimental and Modelling Study of the Selective Noncatalytic Reduction of NO by Ammonia in the Presence of Hydrocarbons, Comb, and Flame 99 (1994) 660. 

The third strategy for minimizing NOx, known as posttreatment, involves removing NOx from the exhaust gases after the NOx has already been formed in the combustion chamber. Two of the most common methods of posttreatment are selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR).7 Wet techniques for posttreatment include oxidation/absorption, oxidation/absorption/reduc-tion, absorption/oxidation, and absorption/reduction. Dry techniques for posttreatment, besides SCR and SNCR, include activated carbon beds, electron beam radiation, and reaction with hydrocarbons. 

Postcombustion NO control options such as selective Noncatalytic reduction (SNCR), SCR, and catalytic oxidation/scrubbing. 

Selective noncatalytic reduction (SNCR) (e.g., Ammonia injection or Thermal DeNOx)... 

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